question about firmware RTC design (for rtcwake)

DISCUSSION Marek Behún 2 participants 6 messages May 22, 2023 – Jun 8, 2023 Real time clock (RTC)

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Marek Behún

author

2023-05-22 14:46 UTC #0 lore raw

Hello RTC guys,

this is a request for suggestions / question about how to implement a RTC driver so that it will be accepted into upstream Linux.

Let me start with the description of the problem.

The MCU on the Turris Omnia router can disable / enable power regulators basically for the whole board, giving capability for platform specific poweroff and for powering the system on at a specified time.

I want to add support for this to the MCU firmware and write a driver for Linux. Since I know only of one utility by which users can request the system to power on at a specified time - the rtcwake utility - I want to write this as a RTC driver.

Because the MCU is not a true RTC in the sense that it does not have battery to count time when not under power, I am wondering how exactly to design this. One problem is that the RTC driver must implement the read function, but the MCU does not know the real time unless it is told beforehand.

Note that the board also has a true RTC with battery, handled by the rtc-armada38x driver. We can make sure that this true RTC is used for hctosys.

Proposal 1:
- the MCU will implement only one command, poweroff, with optional argument wakeup_timeout, the number of seconds after which to power the board up again
- the corresponding driver will register a system off handler and a RTC device
- the RTC will implement methods as:
.read_time will use ktime_get(), so it will return monotonic clock
.set_alarm will store the requested wakeup time in an internal
structure
.alarm_irq_enable will just update the internal structure
.read_alarm will return information from the stored structure
- the system off handler will send the poweroff command and it will take the requested wakeup time from the internal structure and subtract current time (ktime_get()) to get the wakeup_timeout argument for the MCU poweroff command

- advantages:
- MCU needs only to implement one command
- disadvantages:
- the new RTC device will not behave at all like a rtc device regarding timekeeping, since .read_time returns system's monotonic clock. This may confuse users who do not know about the fact that this rtc device is only meant for rtcwake. We can print this info into dmesg, but...
- removing the driver and loading it back loses information about set wakeup time

Proposal 2:
- the MCU will implement:
- command for getting and setting clock that the MCU will count The clock will be considered invalid unless it was set at least once.
- command for getting and setting wakeup alarm
- command for power off
- the corresponding driver will again register a system off handler and a RTC device
- the system off handler will just send the poweroff command
- the RTC device can now behave like a true RTC device, but without a battery, the RTC_VL_BACKUP_EMPTY bit will be set.
The userspace can set time from the true RTC, so that rtcwake may be used

Overall proposal 1 is easier to implement, but the resulting /dev/rtc device will not be a true RTC, which may confuse users. Proposal 2 gives more complexity to MCU firmware.

Personally I prefer proposal one.

Do you guys have suggestions? Which kind of driver would you be willing to accept?

Note that the driver will probably live under drivers/mfd or drivers/platform, since the MCU implements other features, as well (a GPIO controller, for example).

Marek

Alexandre Belloni

maintainer

2023-05-22 19:40 UTC #1 lore raw

Hello,

Quoted 70 lines · Marek Behún

Hello RTC guys,
this is a request for suggestions / question about how to implement a RTC driver so that it will be accepted into upstream Linux.
Let me start with the description of the problem.
The MCU on the Turris Omnia router can disable / enable power regulators basically for the whole board, giving capability for platform specific poweroff and for powering the system on at a specified time.
I want to add support for this to the MCU firmware and write a driver for Linux. Since I know only of one utility by which users can request the system to power on at a specified time - the rtcwake utility - I want to write this as a RTC driver.
Because the MCU is not a true RTC in the sense that it does not have battery to count time when not under power, I am wondering how exactly to design this. One problem is that the RTC driver must implement the read function, but the MCU does not know the real time unless it is told beforehand.
Note that the board also has a true RTC with battery, handled by the rtc-armada38x driver. We can make sure that this true RTC is used for hctosys.
Proposal 1:
- the MCU will implement only one command, poweroff, with optional argument wakeup_timeout, the number of seconds after which to power the board up again
- the corresponding driver will register a system off handler and a RTC device
- the RTC will implement methods as:
.read_time will use ktime_get(), so it will return monotonic clock
.set_alarm will store the requested wakeup time in an internal
structure
.alarm_irq_enable will just update the internal structure
.read_alarm will return information from the stored structure
- the system off handler will send the poweroff command and it will take the requested wakeup time from the internal structure and subtract current time (ktime_get()) to get the wakeup_timeout argument for the MCU poweroff command
- advantages:
- MCU needs only to implement one command
- disadvantages:
- the new RTC device will not behave at all like a rtc device regarding timekeeping, since .read_time returns system's monotonic clock. This may confuse users who do not know about the fact that this rtc device is only meant for rtcwake. We can print this info into dmesg, but...
- removing the driver and loading it back loses information about set wakeup time
Proposal 2:
- the MCU will implement:
- command for getting and setting clock that the MCU will count The clock will be considered invalid unless it was set at least once.
- command for getting and setting wakeup alarm
- command for power off
- the corresponding driver will again register a system off handler and a RTC device
- the system off handler will just send the poweroff command
- the RTC device can now behave like a true RTC device, but without a battery, the RTC_VL_BACKUP_EMPTY bit will be set.
The userspace can set time from the true RTC, so that rtcwake may be used

Quoted 9 lines

Overall proposal 1 is easier to implement, but the resulting /dev/rtc device will not be a true RTC, which may confuse users. Proposal 2 gives more complexity to MCU firmware.
Personally I prefer proposal one.
Do you guys have suggestions? Which kind of driver would you be willing to accept?

You probably need to look at rtc-meson-vrtc.c, rtc-fsl-ftm-alarm.c and rtc-brcmstb-waketimer.c which implement something similar.

Honestly, I would go for an in-between proposal where you would store the requested alarm time (or more likely countdown) on set_alarm/alarm_irq_enable so you would get .read_alarm working.

However, my main concern is that this is yet another custom protocol. We can't possibly have a driver for everyone implementing a timer in their FPGA/CPLD/cortexM.

How will you communicate with the MCU, can't you use an already existing driver?

Regards,

Signature · 3 lines

Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Marek Behún

author

2023-05-22 21:15 UTC #2 lore raw

Quoted 90 lines · On Mon, 22 May 2023 21:40:09 +0200 Alexandre Belloni…

Hello,
Quoted 70 lines · Marek Behún
Hello RTC guys,
this is a request for suggestions / question about how to implement a RTC driver so that it will be accepted into upstream Linux.
Let me start with the description of the problem.
The MCU on the Turris Omnia router can disable / enable power regulators basically for the whole board, giving capability for platform specific poweroff and for powering the system on at a specified time.
I want to add support for this to the MCU firmware and write a driver for Linux. Since I know only of one utility by which users can request the system to power on at a specified time - the rtcwake utility - I want to write this as a RTC driver.
Because the MCU is not a true RTC in the sense that it does not have battery to count time when not under power, I am wondering how exactly to design this. One problem is that the RTC driver must implement the read function, but the MCU does not know the real time unless it is told beforehand.
Note that the board also has a true RTC with battery, handled by the rtc-armada38x driver. We can make sure that this true RTC is used for hctosys.
Proposal 1:
- the MCU will implement only one command, poweroff, with optional argument wakeup_timeout, the number of seconds after which to power the board up again
- the corresponding driver will register a system off handler and a RTC device
- the RTC will implement methods as:
.read_time will use ktime_get(), so it will return monotonic clock
.set_alarm will store the requested wakeup time in an internal
structure
.alarm_irq_enable will just update the internal structure
.read_alarm will return information from the stored structure
- the system off handler will send the poweroff command and it will take the requested wakeup time from the internal structure and subtract current time (ktime_get()) to get the wakeup_timeout argument for the MCU poweroff command
- advantages:
- MCU needs only to implement one command
- disadvantages:
- the new RTC device will not behave at all like a rtc device regarding timekeeping, since .read_time returns system's monotonic clock. This may confuse users who do not know about the fact that this rtc device is only meant for rtcwake. We can print this info into dmesg, but...
- removing the driver and loading it back loses information about set wakeup time
Proposal 2:
- the MCU will implement:
- command for getting and setting clock that the MCU will count The clock will be considered invalid unless it was set at least once.
- command for getting and setting wakeup alarm
- command for power off
- the corresponding driver will again register a system off handler and a RTC device
- the system off handler will just send the poweroff command
- the RTC device can now behave like a true RTC device, but without a battery, the RTC_VL_BACKUP_EMPTY bit will be set.
The userspace can set time from the true RTC, so that rtcwake may be used
Quoted 9 lines
Overall proposal 1 is easier to implement, but the resulting /dev/rtc device will not be a true RTC, which may confuse users. Proposal 2 gives more complexity to MCU firmware.
Personally I prefer proposal one.
Do you guys have suggestions? Which kind of driver would you be willing to accept?
You probably need to look at rtc-meson-vrtc.c, rtc-fsl-ftm-alarm.c and rtc-brcmstb-waketimer.c which implement something similar.
Honestly, I would go for an in-between proposal where you would store the requested alarm time (or more likely countdown) on set_alarm/alarm_irq_enable so you would get .read_alarm working.

Quoted 6 lines

However, my main concern is that this is yet another custom protocol. We can't possibly have a driver for everyone implementing a timer in their FPGA/CPLD/cortexM.
How will you communicate with the MCU, can't you use an already existing driver?

The MCU exposes a command interface over I2C. There already are existing commands, which needs to stay for backwards compatibility.

It is theoretically possible to simulate an existing RTC device on another I2C address, but I would need to study them, because the boards are shipped with three different MCUs (STM32, GD32, NXP's MKL81) and they sometimes have a little different I2C slave behavior.

But I will need to create a platform/mfd driver anyway for the system off handler and GPIO controller. If I am going to create a new driver anyway, why not add the RTC functionality as well?

Marek

Alexandre Belloni

maintainer

2023-06-07 08:28 UTC #3 lore raw

Quoted 17 lines · Marek Behún

Quoted 7 lines
You probably need to look at rtc-meson-vrtc.c, rtc-fsl-ftm-alarm.c and rtc-brcmstb-waketimer.c which implement something similar.
Honestly, I would go for an in-between proposal where you would store the requested alarm time (or more likely countdown) on set_alarm/alarm_irq_enable so you would get .read_alarm working.
Quoted 6 lines
However, my main concern is that this is yet another custom protocol. We can't possibly have a driver for everyone implementing a timer in their FPGA/CPLD/cortexM.
How will you communicate with the MCU, can't you use an already existing driver?
The MCU exposes a command interface over I2C. There already are existing commands, which needs to stay for backwards compatibility.

Quoted 8 lines

It is theoretically possible to simulate an existing RTC device on another I2C address, but I would need to study them, because the boards are shipped with three different MCUs (STM32, GD32, NXP's MKL81) and they sometimes have a little different I2C slave behavior.
But I will need to create a platform/mfd driver anyway for the system off handler and GPIO controller. If I am going to create a new driver anyway, why not add the RTC functionality as well?

No, this is not how MFD is working, you will be writing a separate RTC driver or reusing an existing one. Have a look at the recent isl1208 series:

https://lore.kernel.org/linux-rtc/OS0PR01MB5922DAC377266672ADA9FC28864DA@OS0PR01MB5922.jpnprd01.prod.outlook.com/T/#mab0a75187abf7d8aada2c3517ebfdf7241f4bc7a

This patch adds supports for the isl1208 on board of a PMIC, as you can see, this is a very small change versus a full blown RTC driver.

Signature · 3 lines

Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

Marek Behún

author

2023-06-08 09:06 UTC #4 lore raw

Quoted 23 lines · On Wed, 7 Jun 2023 10:28:28 +0200 Alexandre Belloni…

Quoted 22 lines · Marek Behún
Quoted 7 lines
You probably need to look at rtc-meson-vrtc.c, rtc-fsl-ftm-alarm.c and rtc-brcmstb-waketimer.c which implement something similar.
Honestly, I would go for an in-between proposal where you would store the requested alarm time (or more likely countdown) on set_alarm/alarm_irq_enable so you would get .read_alarm working.
Quoted 6 lines
However, my main concern is that this is yet another custom protocol. We can't possibly have a driver for everyone implementing a timer in their FPGA/CPLD/cortexM.
How will you communicate with the MCU, can't you use an already existing driver?
The MCU exposes a command interface over I2C. There already are existing commands, which needs to stay for backwards compatibility.
It is theoretically possible to simulate an existing RTC device on another I2C address, but I would need to study them, because the boards are shipped with three different MCUs (STM32, GD32, NXP's MKL81) and they sometimes have a little different I2C slave behavior.

Quoted 6 lines

Quoted 3 lines
But I will need to create a platform/mfd driver anyway for the system off handler and GPIO controller. If I am going to create a new driver anyway, why not add the RTC functionality as well?
No, this is not how MFD is working, you will be writing a separate RTC driver or reusing an existing one. Have a look at the recent isl1208

THX for the reply.
I have one I2C client through which I need to implement RTC, GPIO and system power off. I thought such drivers live in drivers/mfd... Am I wrong?

Marek

Quoted 6 lines

series:
https://lore.kernel.org/linux-rtc/OS0PR01MB5922DAC377266672ADA9FC28864DA@OS0PR01MB5922.jpnprd01.prod.outlook.com/T/#mab0a75187abf7d8aada2c3517ebfdf7241f4bc7a
This patch adds supports for the isl1208 on board of a PMIC, as you can see, this is a very small change versus a full blown RTC driver.

Alexandre Belloni

maintainer

2023-06-08 09:26 UTC #5 lore raw

Quoted 30 lines · Marek Behún

Quoted 27 lines · On Wed, 7 Jun 2023 10:28:28 +0200 Alexandre Belloni…
Quoted 25 lines · Marek Behún
Quoted 7 lines
You probably need to look at rtc-meson-vrtc.c, rtc-fsl-ftm-alarm.c and rtc-brcmstb-waketimer.c which implement something similar.
Honestly, I would go for an in-between proposal where you would store the requested alarm time (or more likely countdown) on set_alarm/alarm_irq_enable so you would get .read_alarm working.
Quoted 6 lines
However, my main concern is that this is yet another custom protocol. We can't possibly have a driver for everyone implementing a timer in their FPGA/CPLD/cortexM.
How will you communicate with the MCU, can't you use an already existing driver?
The MCU exposes a command interface over I2C. There already are existing commands, which needs to stay for backwards compatibility.
It is theoretically possible to simulate an existing RTC device on another I2C address, but I would need to study them, because the boards are shipped with three different MCUs (STM32, GD32, NXP's MKL81) and they sometimes have a little different I2C slave behavior.
But I will need to create a platform/mfd driver anyway for the system off handler and GPIO controller. If I am going to create a new driver anyway, why not add the RTC functionality as well?

Quoted 7 lines

Quoted 2 lines
No, this is not how MFD is working, you will be writing a separate RTC driver or reusing an existing one. Have a look at the recent isl1208
THX for the reply.
I have one I2C client through which I need to implement RTC, GPIO and system power off. I thought such drivers live in drivers/mfd... Am I wrong?

You are not wrong but the code doesn't necessarily live in drivers/mfd but is in the subsystems instead.

Quoted 11 lines

Marek
Quoted 6 lines
series:
https://lore.kernel.org/linux-rtc/OS0PR01MB5922DAC377266672ADA9FC28864DA@OS0PR01MB5922.jpnprd01.prod.outlook.com/T/#mab0a75187abf7d8aada2c3517ebfdf7241f4bc7a
This patch adds supports for the isl1208 on board of a PMIC, as you can see, this is a very small change versus a full blown RTC driver.

Signature · 3 lines

Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com